One general said that ending an ISIS reign over Mosul will not end its presence in the country

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An Iraqi Federal Police vehicle passes through a checkpoint in Qayara, some 31 miles south of Mosul, Iraq, Wednesday, Oct. 26, 2016.

A senior U.S. military official warns that ISIS fighters in Mosul are likely to resort to even more extreme actions as they face more military pressure on the city.

"Unfortunately, ISIS will likely grow more oppressive to the population in areas they control," the official said, adding, "they will either leave or they will be more oppressive and force folks to fight that shouldn't fight."